_content/doc/tutorial/generics.md

<!--{
  "Title": "Tutorial: Getting started with generics",
  "Breadcrumb": true
}-->

This tutorial introduces the basics of generics in Go. With generics, you can
declare and use functions or types that are written to work with any of a set
of types provided by calling code.

In this tutorial, you'll declare two simple non-generic functions, then capture
the same logic in a single generic function.

You'll progress through the following sections:

1. Create a folder for your code.
2. Add non-generic functions.
3. Add a generic function to handle multiple types.
4. Remove type arguments when calling the generic function.
5. Declare a type constraint.

**Note:** For other tutorials, see [Tutorials](/doc/tutorial/index.html).

## Prerequisites

*   **Go.** We recommend using the latest version of Go to follow this tutorial.
    For installation instructions, see [Installing Go](/doc/install).
*   **A tool to edit your code.** Any text editor you have will work fine.
*   **A command terminal.** Go works well using any terminal on Linux and Mac,
    and on PowerShell or cmd in Windows.

## Create a folder for your code {#create_folder}

To begin, create a folder for the code you’ll write.

1. Open a command prompt and change to your home directory.

    On Linux or Mac:

    ```
    $ cd
    ```

    On Windows:

    ```
    C:\> cd %HOMEPATH%
    ```

    The rest of the tutorial will show a $ as the prompt. The commands you use
    will work on Windows too.

2. From the command prompt, create a directory for your code called generics.

    ```
    $ mkdir generics
    $ cd generics
    ```

3. Create a module to hold your code.

    Run the `go mod init` command, giving it your new code’s module path.

    ```
    $ go mod init example/generics
    go: creating new go.mod: module example/generics
    ```

    **Note:** For production code, you’d specify a module path that’s more specific
    to your own needs. For more, be sure to see
    [Managing dependencies](/doc/modules/managing-dependencies).

Next, you'll add some simple code to work with maps.

## Add non-generic functions {#non_generic_functions}

In this step, you'll add two functions that each add together the values of a
map and return the total.

You're declaring two functions instead of one because you're working with two
different types of maps: one that stores `int64` values, and one that stores `float64` values.

#### Write the code

1. Using your text editor, create a file called main.go in the generics
    directory. You’ll write your Go code in this file.
2. Into main.go, at the top of the file, paste the following package
    declaration.

    ```
    package main
    ```

    A standalone program (as opposed to a library) is always in package `main`.

3. Beneath the package declaration, paste the following two function
    declarations.

    ```
    // SumInts adds together the values of m.
    func SumInts(m map[string]int64) int64 {
    	var s int64
    	for _, v := range m {
    		s += v
    	}
    	return s
    }

    // SumFloats adds together the values of m.
    func SumFloats(m map[string]float64) float64 {
    	var s float64
    	for _, v := range m {
    		s += v
    	}
    	return s
    }
    ```

    In this code, you:

    *   Declare two functions to add together the values of a map and return
        the sum.
        *   `SumFloats` takes a map of `string` to `float64` values.
        *   `SumInts` takes a map of `string` to `int64` values.

4. At the top of main.go, beneath the package declaration, paste the following
    `main` function to initialize the two maps and use them as arguments when
    calling the functions you declared in the preceding step.

    ```
    func main() {
    	// Initialize a map for the integer values
    	ints := map[string]int64{
    		"first":  34,
    		"second": 12,
    	}

    	// Initialize a map for the float values
    	floats := map[string]float64{
    		"first":  35.98,
    		"second": 26.99,
    	}

    	fmt.Printf("Non-Generic Sums: %v and %v\n",
    		SumInts(ints),
    		SumFloats(floats))
    }
    ```

    In this code, you:

    *   Initialize a map of `float64` values and a map of `int64` values, each
        with two entries.
    *   Call the two functions you declared earlier to find the sum of each
        map's values.
    *   Print the result.

5. Near the top of main.go, just beneath the package declaration, import the
    package you’ll need to support the code you’ve just written.

    The first lines of code should look like this:

    ```
    package main

    import "fmt"
    ```

6. Save main.go.

#### Run the code

From the command line in the directory containing main.go, run the code.

```
$ go run .
Non-Generic Sums: 46 and 62.97
```

With generics, you can write one function here instead of two. Next, you’ll
add a single generic function for maps containing either integer or float values.

## Add a generic function to handle multiple types {#add_generic_function}

In this section, you'll add a single generic function that can receive a map
containing either integer or float values, effectively replacing the two
functions you just wrote with a single function.

To support values of either type, that single function will need a way to
declare what types it supports. Calling code, on the other hand, will need a
way to specify whether it is calling with an integer or float map.

To support this, you'll write a function that declares _type parameters_ in
addition to its ordinary function parameters. These type parameters make the
function generic, enabling it to work with arguments of different types. You'll
call the function with _type arguments_ and ordinary function arguments.

Each type parameter has a _type constraint_ that acts as a kind of meta-type
for the type parameter. Each type constraint specifies the permissible type
arguments that calling code can use for the respective type parameter.

While a type parameter's constraint typically represents a set of types, at
compile time the type parameter stands for a single type – the type provided
as a type argument by the calling code. If the type argument's type isn't
allowed by the type parameter's constraint, the code won't compile.

Keep in mind that a type parameter must support all the operations the generic
code is performing on it. For example, if your function's code were to try to
perform `string` operations (such as indexing) on a type parameter whose
constraint included numeric types, the code wouldn't compile.

In the code you're about to write, you'll use a constraint that allows either
integer or float types.

#### Write the code

1. Beneath the two functions you added previously, paste the following generic
    function.

    ```
    // SumIntsOrFloats sums the values of map m. It supports both int64 and float64
    // as types for map values.
    func SumIntsOrFloats[K comparable, V int64 | float64](m map[K]V) V {
        var s V
        for _, v := range m {
            s += v
        }
        return s
    }
    ```

    In this code, you:

    *   Declare a `SumIntsOrFloats` function with two type parameters (inside
        the square brackets), `K` and `V`, and one argument that uses the type
        parameters, `m` of type `map[K]V`. The function returns a value of
        type `V`.
    *   Specify for the `K` type parameter the type constraint `comparable`.
        Intended specifically for cases like these, the `comparable` constraint
        is predeclared in Go. It allows any type whose values may be used as an
        operand of the comparison operators `==` and `!=`. Go requires that map
        keys be comparable. So declaring `K` as `comparable` is necessary so you
        can use `K` as the key in the map variable. It also ensures that calling
        code uses an allowable type for map keys.
    *   Specify for the `V` type parameter a constraint that is a union of two
        types: `int64` and `float64`. Using `|` specifies a union of the two
        types, meaning that this constraint allows either type. Either type
        will be permitted by the compiler as an argument in the calling code.
    *   Specify that the `m` argument is of type `map[K]V`, where `K` and `V`
        are the types already specified for the type parameters. Note that we
        know `map[K]V` is a valid map type because `K` is a comparable type. If
        we hadn’t declared `K` comparable, the compiler would reject the
        reference to `map[K]V`.

2. In main.go, beneath the code you already have, paste the following code.

    ```
    fmt.Printf("Generic Sums: %v and %v\n",
    	SumIntsOrFloats[string, int64](ints),
    	SumIntsOrFloats[string, float64](floats))
    ```

    In this code, you:

    *   Call the generic function you just declared, passing each of the maps
        you created.
    *   Specify type arguments – the type names in square brackets – to be
        clear about the types that should replace type parameters in the
        function you're calling.

        As you'll see in the next section, you can often omit the type
        arguments in the function call. Go can often infer them from your code.
    *   Print the sums returned by the function.

#### Run the code

From the command line in the directory containing main.go, run the code.

```
$ go run .
Non-Generic Sums: 46 and 62.97
Generic Sums: 46 and 62.97
```

To run your code, in each call the compiler replaced the type parameters with
the concrete types specified in that call.

In calling the generic function you wrote, you specified type arguments that
told the compiler what types to use in place of the function's type parameters.
As you'll see in the next section, in many cases you can omit these type
arguments because the compiler can infer them.

## Remove type arguments when calling the generic function {#remove_type_arguments}

In this section, you'll add a modified version of the generic function call,
making a small change to simplify the calling code. You'll remove the type
arguments, which aren't needed in this case.

You can omit type arguments in calling code when the Go compiler can infer the
types you want to use. The compiler infers type arguments from the types of
function arguments.

Note that this isn't always possible. For example, if you needed to call a
generic function that had no arguments, you would need to include the type
arguments in the function call.

#### Write the code

*   In main.go, beneath the code you already have, paste the following code.

    ```
    fmt.Printf("Generic Sums, type parameters inferred: %v and %v\n",
    	SumIntsOrFloats(ints),
    	SumIntsOrFloats(floats))
    ```

    In this code, you:

    *   Call the generic function, omitting the type arguments.

#### Run the code

From the command line in the directory containing main.go, run the code.

```
$ go run .
Non-Generic Sums: 46 and 62.97
Generic Sums: 46 and 62.97
Generic Sums, type parameters inferred: 46 and 62.97
```

Next, you'll further simplify the function by capturing the union of integers
and floats into a type constraint you can reuse, such as from other code.

## Declare a type constraint {#declare_type_constraint}

In this last section, you'll move the constraint you defined earlier into its
own interface so that you can reuse it in multiple places. Declaring
constraints in this way helps streamline code, such as when a constraint is
more complex.

You declare a _type constraint_ as an interface. The constraint allows any
type implementing the interface. For example, if you declare a type constraint
interface with three methods, then use it with a type parameter in a generic
function, type arguments used to call the function must have all of those
methods.

Constraint interfaces can also refer to specific types, as you'll see in this
section.

#### Write the code

1. Just above `main`, immediately after the import statements, paste the
    following code to declare a type constraint.

    ```
    type Number interface {
        int64 | float64
    }
    ```

    In this code, you:

    *   Declare the `Number` interface type to use as a type constraint.
    *   Declare a union of `int64` and `float64` inside the interface.

        Essentially, you're moving the union from the function declaration
        into a new type constraint. That way, when you want to constrain a type
        parameter to either `int64` or `float64`, you can use this `Number`
        type constraint instead of writing out `int64 | float64`.

2. Beneath the functions you already have, paste the following generic
    `SumNumbers` function.

    ```
    // SumNumbers sums the values of map m. It supports both integers
    // and floats as map values.
    func SumNumbers[K comparable, V Number](m map[K]V) V {
        var s V
        for _, v := range m {
            s += v
        }
        return s
    }
    ```

    In this code, you:

    *   Declare a generic function with the same logic as the generic function
        you declared previously, but with the new interface type instead of the
        union as the type constraint. As before, you use the type parameters
        for the argument and return types.

3. In main.go, beneath the code you already have, paste the following code.

    ```
    fmt.Printf("Generic Sums with Constraint: %v and %v\n",
    	SumNumbers(ints),
    	SumNumbers(floats))
    ```

    In this code, you:

    *   Call `SumNumbers` with each map, printing the sum from the values of
        each.

        As in the preceding section, you omit the type arguments (the type
        names in square brackets) in calls to the generic function. The Go
        compiler can infer the type argument from other arguments.

#### Run the code

From the command line in the directory containing main.go, run the code.

```
$ go run .
Non-Generic Sums: 46 and 62.97
Generic Sums: 46 and 62.97
Generic Sums, type parameters inferred: 46 and 62.97
Generic Sums with Constraint: 46 and 62.97
```

## Conclusion {#conclusion}

Nicely done! You've just introduced yourself to generics in Go.

Suggested next topics:

*   The [Go Tour](/tour/) is a great step-by-step
    introduction to Go fundamentals.
*   You'll find useful Go best practices described in
    [Effective Go](/doc/effective_go) and
    [How to write Go code](/doc/code).

## Completed code {#completed_code}

You can run this program in the
[Go playground](/play/p/apNmfVwogK0). On the
playground simply click the **Run** button.

```
package main

import "fmt"

type Number interface {
	int64 | float64
}

func main() {
	// Initialize a map for the integer values
	ints := map[string]int64{
		"first": 34,
		"second": 12,
	}

	// Initialize a map for the float values
	floats := map[string]float64{
		"first": 35.98,
		"second": 26.99,
	}

	fmt.Printf("Non-Generic Sums: %v and %v\n",
		SumInts(ints),
		SumFloats(floats))

	fmt.Printf("Generic Sums: %v and %v\n",
		SumIntsOrFloats[string, int64](ints),
		SumIntsOrFloats[string, float64](floats))

	fmt.Printf("Generic Sums, type parameters inferred: %v and %v\n",
		SumIntsOrFloats(ints),
		SumIntsOrFloats(floats))

	fmt.Printf("Generic Sums with Constraint: %v and %v\n",
		SumNumbers(ints),
		SumNumbers(floats))
}

// SumInts adds together the values of m.
func SumInts(m map[string]int64) int64 {
	var s int64
	for _, v := range m {
		s += v
	}
	return s
}

// SumFloats adds together the values of m.
func SumFloats(m map[string]float64) float64 {
	var s float64
	for _, v := range m {
		s += v
	}
	return s
}

// SumIntsOrFloats sums the values of map m. It supports both floats and integers
// as map values.
func SumIntsOrFloats[K comparable, V int64 | float64](m map[K]V) V {
	var s V
	for _, v := range m {
		s += v
	}
	return s
}

// SumNumbers sums the values of map m. It supports both integers
// and floats as map values.
func SumNumbers[K comparable, V Number](m map[K]V) V {
	var s V
	for _, v := range m {
		s += v
	}
	return s
}